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Full text of "Handbook Of Chemical Engineering - I"

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MECHANICAL SEPARATION                             291
increase the rate of gravity nitration and is often particularly desirable where suction or pressure has the tendency to cause the solids to pass through the filtering medium.
Vacuum Filters, Continuous Type.-Vacuum leaf filters which dipped into a tank or basin of the material to be filtered were the predecessors in point of time of the continuous rotating vacuum filters of today, practically the only type of continuous vacuum filter which exists today. The old leaf type of vacuum filter was intermittent in operation and one of two sets of operations were necessary with them (omitting for brevity the washing pperations). In one set the leaves were stationary and after the tank or basin was filled suction was applied and a cake of the proper thickness accumulated. It was then necessary to run off the balance of the material in the tank or basin, reverse the air current and blow off and wash out the cake which had accumulated. After this was done fresh material was allowed to enter and the cycle of operations was repeated. In the other set of operations and better from a mechanical point of view the filter leaves with their accumulated cake were removed by machinery and after the cake was dropped the leaves could be returned to the tank for further solid accumulation.
The open-tank suction filter is particularly adapted to cases where the amount of solids to be handled is only a small percentage of the total bulk; where the solids are very finely divided and where the cake takes a long while to build. There is but little wear on the filter cloth, and the cake builds to an even resistance, so that it washes evenly and with a minimum of washing.
According to the Industrial Filtration Corporation the rotary filter is indicated when the slurry will permit a cake J in. or more in thickness to build in 2 to 3 minutes when poured into a Buchner filter under 25 in. of suction.
The same corporation builds a hopper dewaterer for granular materials that will not form a cake. In this case a series of hoppers with filter bottoms pass successively under an overhead feed. The granular material is then dewatered by suction, may then be washed if desirable, and finally dumps underneath by gravity.
Figures 15 and 16 show the Portland filter which is similar in principle to other rotary filters and in general arrangement to the Oliver filter. The filter and its operation can be briefly described as follows: The material to be filtered is discharged into the wedge-shaped tank which supports the revolving drum. The air pipes at A} Fig. 15, blow air through the material to stir up the solids after a shut down. The connection to the valve plate at the center of the filter and above the pipe connections are not shown. Figure 16 shows a sprocket wheel near the drive pulley which drives a paddle agitator located in the bottom of the tank. In the interior of the drum will be seen the pipes which run from the panels forming the face of the drum and which are all connected in to the valve plate located at B, Fig. 15. The valve plate is shown dia-grammatically in Fig. 17. A is the plate which is affixed to and revolves with the filter drum. It has as many holes in it as there are filter panels in the filter drum. A. shows 22 holes which would correspond to the number of panels in a filter 12 ft. in diameter. B is the stationary port plate, fitting into a casting divided off into as many compartments as there are port groups. The openings at the front face of this casting are provided with pipe connections leading to vacuum and compression apparatus. For the majority of uses the most satisfactory machines for creating vacuum and compression are small compressors. To obtain vacuum the vacuum piping is connected to the intake end of one compressor and to obtain compression the compression-piping system is connected to the discharge of the other compressor. Compressors with easily removable or flat spring valves mounted in removable valve